Disk drives, particularly fixed disk drives, are valued on the basis of several factors including size (i.e. sometimes referred to as "form factor"), data storage capacity, random access times between data fields located in disparate concentric data tracks of the storage disk (i.e. sometimes referred to as "access time" or "average access time"), cost per byte stored, and useful life (i.e. sometimes rated as "mean time between failures").
When data tracks are arranged as concentric circles on a circular storage surface, outer tracks or circles are longer and therefore have more useful magnetic storage domains than inner tracks. Also, when storage disks are rotated at a constant angular velocity, the data transducer head "flies" at a faster and somewhat higher altitude above outer storage tracks where relative head to disk velocity is greater, than inner tracks.
One known way to increase data storage capacity is to divide the data storage surface into radial zones of tracks, and to optimize data transfer rate to the smallest track (innermost track) within each particular zone. This approach is sometimes called "zoned data recording". The number of data sectors or fields within each track typically may vary from zone to zone. In order to switch from zone to zone, it is necessary for the disk drive to adapt itself in real time to the number of data sectors and to the new data rate.
Other known ways to increase data storage capacity include varying disk rotation in function of radial position of the data transducer head while maintaining data transfer rate substantially constant, as is the case with optical disk technology; and, varying data transfer rate with each track in function of the radial position of the head, while maintaining disk rotation constant, as is the typical case with fixed disk, "flying head" technology.
Issues confronting the designer of a disk drive include head positioning, and data block transfer. Head positioning is typically carried out by a head positioner servo mechanism and involves "track seeking operations" for moving the data transducer head from a departure track to a destination track throughout the radial extent of the storage area of the disk; and, "track following operations" for causing the data transducer head to follow precisely a particular data track during data block reading/writing operations. In order to provide precise head positioning, during both seeking and following, some servo information must be provided to the head positioner servo. This information may be contained on a special data surface written exclusively with servo information, (called a "dedicated servo surface"), or it may be externally supplied as by an optical encoder coupled to the head positioner arm, or it may be supplied from servo information interspersed or "embedded" among the data fields within each data track. One other approach worth mentioning is provided by the open loop stepper motor head positioner servo wherein positional stability of the head at each selected data track location is provided by electromagnetic detents of the stepper motor.
When servo information is embedded on a data surface having zoned data recording, complications arise in reliably providing robust servo head position information. There must be sufficient embedded information to provide stability to the servo loop and to provide position feedback during high speed portions of track seeking operations, so that velocity or position profiles may be adjusted, based on present head velocity or position at the time of the sample. If the servo information is recorded at the same data rate, and in positional relationship with the data blocks, as has been conventionally employed, the servo architecture is complex in the sense of having to switch data rate and servo position, as zones are crossed over. One example of data zones with switched servo sector locations is provided by the disclosure of Ottesen U.S. Pat. No. 4,016,603, for example. If the servo information is regularly spaced radially across the data storage surface and splits at least some of the data fields into segments, complications arise in reading each split data field as a single data block without error. Also, the disk rotational velocity must be monitored and carefully maintained at a predetermined constant angular velocity.
Data fields are conventionally managed by a data sequencer. One example of a data sequencer is to be found in commonly assigned U.S. Pat. No. 4,819,153, the disclosure thereof being incorporated herein by reference. The data sequencer may include an encoder/decoder for transforming non-return-to-zero (NRZ) data into a coded data format, such as a three to two 1,7 run-length-limited (RLL) code in order to achieve compression of data relative to flux transition density on the data surface. (1,7 RLL coding is based on three cede bits or groups for two unencoded data bits, but results in a four to three overall data compaction rate and therefore permits more data to be recorded on the disk per the number of flux transitions that may be contained within the magnetic storage domains.)
A data sequencer conventionally performs the task of decoding data sector overhead information in order to locate a desired storage location, and to obtain information relating to the correctness or validity of data read back from the storage location. Typically implemented as a state machine, a data sequencer conventionally monitors incoming data flow to locate a data identification (ID) preamble field, a data ID address mark, a sector ID field, the data field itself, and usually some small number of error correction syndrome bytes appended to the end of the data field. The sequencer causes appropriate action to be taken when each of the fields is located. For example, if a data block from the data field of a particular track and sector is being sought, the sequencer compares incoming sector ID field information with the sought-after sector information stored in a register. When a positive comparison occurs, the sequencer causes the bytes read from the data field via a data transducer head and a read channel to be sent into a block buffer memory, and the error correction syndrome remainder bytes to be checked. If there are no detected errors in the data bytes as determined by analyzing the error correction code (ECC) remainder bytes, the block is then sent from the buffer memory to the host computer via a suitable interface, such as Small Computer System Interface (SCSI).
In the disk drive described in a commonly assigned U.S. Pat. No. 4,669,004, each sector was handled individually in response to specific input from the supervisory microcontroller. The disclosure of this patent is incorporated herein by reference. As a particular sector was read, the microcontroller would inform the sequencer whether or not to read the next data sector. This microcontroller intervention occurred for every sector.
In a later design, of which the disclosure found in commonly assigned U.S. Pat. No. 5,005,089, is incorporated herein by reference, a programmable sector counter was preset by the microcontroller to a desired sector count, and the sequencer then processed sectors sequentially until the count in the sector counter was reached. The disk drive examples found in the referenced '004 and '153 patents did not include the complication of zoned data recording and split data fields, and the '089 patent did not include embedded servo sectors. Positioner stability in the '089 disk drive example was provided by an optical encoder coupled between a rotary head positioner and the drive base, as was the case of the disk drive example described in the referenced '153 patent.
Heretofore, while split data recording schemes have been proposed in the prior art, recent proposals have typically tasked the data microcontroller with the responsibility for managing each split data field layout in real time, leading to a tremendous level of bus traffic between the microcontroller and the data sequencer during data read and write operations, and precluding the microcontroller from performing other very useful tasks, such as those related to head position servo supervision, error correction, command and status exchanges with the host computer over the interface bus structure, to cite a few examples. These prior approaches have therefore required a separate data transfer microprocessor, meaning that at least two microprocessors were required to implement an overall disk drive architecture.